Pressures and Temperatures of Ultrahigh-Pressure Metamorphism: Implications for UHP Tectonics and H₂O in Subducting Slabs

Kyanite-phengite eclogites can yield robust P-T equilibration conditions that do not depend on the ferrous/ferric ratios of the constituent minerals. Even rocks lacking kyanite or phengite—for which the ferrous/ferric ratios are calculated stoichiometrically—can yield equilibration conditions that differ by < 0.4 GPa. The range of robust P-T conditions for kyanite-phengite UHP eclogites is more restricted than that reported for eclogites generally, with most in the range 600-800°C and < 4 GPa; none fall in the "forbidden zone" defined by geothermal gradients of ≤ 5 K/km. Most UHP eclogites equilibrated at temperatures below the H2O-saturated solidus for crustal rocks. This is probably because hotter UHP terranes are sufficiently low in viscosity that they mix with their surroundings and lose their identity or re-equilibrate continuously during cooling until the solidus is reached. Lawsonite and chloritoid are not known from UHP eclogites.